Wireless communication systems following Universal Mobile Telecommunications Systems (UMTS) technology, were developed as part of Third Generation (3G) Radio Systems, and are maintained by the Third Generation Partnership Project (3GPP). A typical UMTS system architecture in accordance with current 3GPP specifications is depicted in FIG. 1. The UMTS network architecture includes a Core Network (CN) interconnected with a UMTS Terrestrial Radio Access Network (UTRAN) via an Iu interface. The UTRAN is configured to provide wireless telecommunication services to users through wireless transmit receive units (WTRUs), referred to as user equipments (UEs), via a Uu radio interface. A commonly employed air interface defined in the UMTS standard is wideband code division multiple access (W-CDMA). The UTRAN has one or more radio network controllers (RNCs) and base stations, referred to as Node Bs, which collectively provide for the geographic coverage for wireless communications with UEs. Uplink (UL) communications refer to transmissions from UE to Node B, and downlink (DL) communications refer to transmissions from Node B to UE. One or more Node Bs are connected to each RNC via an Iub interface; RNCs within a UTRAN communicate via an Iur interface.
According to 3GPP standard Release 6 for high speed uplink packet access (HSUPA), the MAC layer multiplexes higher layer data into MAC-e PDUs. In a transmission time interval (TTI), the MAC layer sends one MAC-e PDU to the PHY layer to be transmitted over the enhanced dedicated channel (E-DCH) dedicated physical data control channel (E-DPDCH). As part of link adaptation, the MAC layer performs enhanced transport format combination (E-TFC) selection based on radio link control (RLC) logical channel priority, RLC buffer occupancy, physical channel conditions, serving grants, non-serving grants, power limitations, hybrid automatic repeat request (HARQ) profile and logical channel multiplexing.
According to 3GPP standard Release 6, the radio link control (RLC) layer in acknowledged mode (AM) can only operate using fixed RLC protocol data unit (PDU) sizes. In addition, the high-speed medium access control (MAC-hs) entity in the Node B and the medium access control (MAC-e/es) entity in the UE do not support segmentation of the service data units (SDUs) from higher layers. These restrictions may result in performance limitations, especially as high speed packet access (HSPA) evolves towards higher data rates. In order to reach higher data rates and reduce protocol overhead and padding, a number of new features were introduced to the layer 2 (L2) protocol in 3GPP Release 7. In particular, flexible RLC PDU sizes and MAC segmentation in the downlink were introduced. However, corresponding L2 enhancements were not introduced for uplink operation in 3GPP Release 7.
More recently, a new 3GPP work item has been proposed for Improved L2 Uplink to introduce enhancements to L2 operation in the uplink. Some of the objectives of Improved L2 Uplink include: support for flexible RLC PDU sizes; support for MAC segmentation of higher layer PDUs including MAC-d and MAC-c PDUs; smooth transition between old and new protocol formats; and support for seamless state transitions between the CELL_DCH, CELL_FACH, CELL_PCH and URA_PCH states, dependent on potential enhancements to the CELL_FACH uplink transmission.
According to 3GPP Release 7, two MAC sub-layers, MAC-e and MAC-es, handle the enhanced dedicated transport channel (E-DCH) in the uplink. MAC-es sits on top of MAC-e and receives dedicated MAC (MAC-d) PDUs directly from the MAC-d entity. MAC-es SDUs (i.e. MAC-d PDUs) of the same size, coming from a particular logical channel are multiplexed together into a single MAC-es payload, and there is only one MAC-es PDU per logical channel per transmission time interval (TTI) since only one MAC-d PDU size is allowed per logical channel per TTI. A MAC-es header is prepended to the MAC-es payload. The number of PDUs N and a data description indicator (DDI) value identifying the logical channel, the MAC-d flow and the MAC-es SDU size are included as part of the MAC-e header. In case sufficient space is left in the E-DCH transport block or if Scheduling Information (SI) needs to be transmitted, an SI is included at the end of the MAC-e PDU. Multiple MAC-es PDUs from multiple logical channels can be included, but only one MAC-e PDU can be transmitted in a TTI.
According to 3GPP Release 7, all MAC-d PDUs contained in a MAC-es PDU are fixed to a preconfigured PDU size. In contrast, according to the Improved L2 Uplink work item, a MAC-es PDU may contain one or more MAC-d PDUs, MAC-c PDUs or RLC PDUs, or segments thereof, of different sizes as received from higher layers. The existing MAC-e/es headers and protocols according to 3GPP Release 7 or earlier do not support such flexibility in MAC-es SDU sizes. For example, the data description indicator (DDI) field that indicates the Logical channel ID, MAC-d flow ID, and the PDU size can no longer be used, because the MAC-es PDU size will not be from a set of fixed sizes. More generally, introduction of segmentation at the MAC layer increases the complexity in designing a low-overhead process for the MAC-e/es header. Therefore, it is desirable to have efficient methods of specifying the lengths of MAC-es SDUs, the logical channel it belongs to, and how it is segmented when flexible RLC PDU sizes and MAC segmentation are permitted.